Update generator OWNERS

I reviewed PRs over the last year:
- nikhita has done consistent review on almost every change
  that touches this code, added a number of substantive
  changes herself, and is a contribex tech lead
- pruning grodrigues3 due to lack of activity in the past year

generator/OWNERS

@@ -2,12 +2,12 @@
 
 reviewers:
   - cblecker
-  - grodrigues3
   - jamiehannaford
+  - nikhita
   - spiffxp
 approvers:
   - cblecker
-  - grodrigues3
+  - nikhita
   - spiffxp
 labels:
   - sig/contributor-experience