Update matrix doc header titles for replace/modify/append (#2083)

Co-authored-by: Ben Broderick Phillips <[email protected]>

eng/common/scripts/job-matrix/README.md

@@ -14,7 +14,7 @@
     * [include/exclude](#includeexclude)
     * [displayNames](#displaynames-1)
     * [Filters](#filters)
-    * [Replace](#replace-values)
+    * [Replace/Modify/Append](#replacemodifyappend-values)
     * [NonSparseParameters](#nonsparseparameters)
     * [Under the hood](#under-the-hood)
 * [Testing](#testing)
@@ -169,7 +169,7 @@ To import a matrix, add a parameter with the key `$IMPORT`:
 
 Importing can be useful, for example, in cases where there is a shared base matrix, but there is a need to run it
 once for each instance of a language version. Importing does not support overriding duplicate parameters. To achieve
-this, use the [Replace](#replace-values) argument instead.
+this, use the [Replace](#replacemodifyappend-values) argument instead.
 
 The `Selection` and `NonSparseParameters` parameters are respected when generating an imported matrix.
 
@@ -399,7 +399,7 @@ named "ExcludedKey", a framework variable containing either "461" or "5.0", and
   -Filters @("ExcludedKey=^$", "framework=(461|5\.0)", "SupportedClouds=^$|.*Public.*")
 ```
 
-#### Replace values
+#### Replace/Modify/Append Values
 
 Replacements for values can be passed to the matrix as an array of strings, each matching the format of `<keyRegex>=<valueRegex>/<replacementValue>`.
 The replace argument will find any permutations where the key fully matches the key regex and the value fully matches the value regex, and replace the value with