Remove trailing whitespace.

Most of this came from places where the RST to markdown export process
included indentation on blank lines in comments.

After this commit, I confirmed that there's no remaining trailing
whitespace by running:

    git grep -I ' $'

Confirmed that there's no change in the rendered content.

Author: amartini51

Sources/TSPL/TSPL.docc/GuidedTour/Compatibility.md

@@ -9,7 +9,7 @@ that are written in either Swift 5.7, Swift 4.2, or Swift 4.
 
 <!--
   - test: `swift-version`
-  
+
   ```swifttest
   >> #if swift(>=5.7.1)
   >>     print("Too new")