Fix a typo in ${NEW} variable in Post-processing tools for diagnostics section

While reading up I realized there is a typo in: `how to use  utils/process-stats-dir.py`. The example asks to use OLD and NEW compiler but the documentation uses only ${OLD} compiler in example

Author: prrane

docs/CompilerPerformance.md

@@ -793,7 +793,7 @@ performance between two compilers, say `${OLD}/swiftc` and `${NEW}/swiftc`:
 ```
 $ mkdir stats-old stats-new
 $ ${OLD}/swiftc -stats-output-dir stats-old test.swift
-$ ${OLD}/swiftc -stats-output-dir stats-new test.swift
+$ ${NEW}/swiftc -stats-output-dir stats-new test.swift
 $ utils/process-stats-dir.py --compare-stats-dirs stats-old stats-new
 old     new     delta_pct       name
 1402939 1430732 1.98    AST.NumASTBytesAllocated