Add the 'is' in "of 2 spaces is used"

styleguide.md

@@ -19,7 +19,7 @@ The document is in ongoing transition to _one sentence per line_ source code for
 This means that any modified or new text should have each sentence alone on a single physical line in the source file.
 Once we have the physical line breaks in the correct places, the diffs of future changes will become clean and easy to grasp.
 
-When indenting the contents of a LaTeX environment, an indentation of 2 spaces used.
+When indenting the contents of a LaTeX environment, an indentation of 2 spaces is used.
 It is recommended to not add indentation before `\item`:
 ```
 \begin{itemize}