Re #7469: doc: typo found by deciphering sphinx warning

Wasn't immediate to get to the error from the warning
```
doc/cabal-project.rst:2: WARNING: Duplicate ID: "cfg-field-type".
```
(wrong line number, "cfg-field-type" is not greppable).

(cherry picked from commit 0cee02f)

doc/cabal-project.rst

@@ -182,7 +182,7 @@ This allows sharing of packages across different projects.
 
 .. cfg-field:: location: VCS location (usually URL)
 
-.. cfg-field:: type: VCS tag
+.. cfg-field:: tag: VCS tag
 
 .. cfg-field:: subdir: subdirectory list