doc: Use more common spelling of "style guide"

See
[trend](https://trends.google.com/explore?q=styleguide%2Cstyle%2520guide&date=all&geo=Worldwide)
for one indication that the new spelling is much more common.

ci/github-script/reminders/docs-styleguide.md

@@ -1,6 +1,6 @@
 Thanks for contributing to the documentation
 
-Make sure you follow the [documentation styleguide](https://github.com/NixOS/nixpkgs/blob/master/doc/styleguide.md), most notably:
+Make sure you follow the [documentation style guide](https://github.com/NixOS/nixpkgs/blob/master/doc/styleguide.md), most notably:
 
 - Show, don't tell: lead with a minimal working example; explanation follows the code.
 - No meta-commentary: don't write "This section explains how to…", just do it.

doc/README.md

@@ -13,7 +13,7 @@ When writing new docs use **Progressive Disclosure:**
 - Use **examples** first to show how to get something done.
 - Keep **explanation** lean.
 
-Use our [styleguide](./styleguide.md) for more in depth guidance on writing good documentation.
+Use our [style guide](./styleguide.md) for more in depth guidance on writing good documentation.
 
 Documentation about Nixpkgs belongs here, this includes 'getting-started'-guides and 'onboarding-guides' for *using* Nixpkgs and the language frameworks it ships.
 
@@ -216,7 +216,7 @@ When needed, each convention explains why it exists, so you can make a decision
 Note that these conventions are about the **structure** of the manual (and its source files), not about the content that goes in it.
 You, as the writer of documentation, are still in charge of its content.
 
-**For prose style, see the [documentation styleguide](./styleguide.md).**
+**For prose style, see the [documentation style guide](./styleguide.md).**
 
 ### Document structure
 
@@ -286,7 +286,7 @@ When changing existing content, update formatting if possible, but avoid excessi
 
 ### Examples first
 
-Put examples before detailed explanations (see the [styleguide](./styleguide.md) for the rationale).
+Put examples before detailed explanations (see the [style guide](./styleguide.md) for the rationale).
 
 Use this structure for each documented item:
 

doc/styleguide.md

@@ -1,4 +1,4 @@
-# Styleguide
+# Style guide
 
 Use this page as a reference and style guide for our internal and external documentation.