Fix typo (refs: #15999)

I want a search'n'replace feature that also updates grammar accordingly.
M-x what?

wiki/src/contribute/build/reproducible.mdwn

@@ -57,8 +57,8 @@ the need to go through this step decreases our time to remediation for
 fixing security issues, and makes it easier for developers with poor
 access to the Internet to take care of a release.
 
-Build and compare a Tails ISO and USB images
-============================================
+Build and compare Tails ISO and USB images
+==========================================
 
 Build Tails ISO and USB images
 ------------------------------