Fix typo (argoproj#20127)

Remove a redundant "is".

Signed-off-by: Symeon Meichanetzoglou <simosmeih@gmail.com>

docs/user-guide/diff-strategies.md

@@ -18,7 +18,7 @@ Argo CD currently has 3 different strategies to calculate diffs:
 ## Structured-Merge Diff
 *Current Status: [Beta][1] (Since v2.5.0)*
 
-This is diff strategy is automatically used when Server-Side Apply
+This diff strategy is automatically used when Server-Side Apply
 sync option is enabled. It uses the [structured-merge-diff][2] library
 used by Kubernetes to calculate diffs based on fields ownership. There
 are some challenges using this strategy to calculate diffs for CRDs