chore: update keywords

PR-URL: #12830
Reviewed-by: Athan Reines <kgryte@gmail.com>

Author: Planeshifter

lib/node_modules/@stdlib/stats/base/dists/negative-binomial/mean/package.json

@@ -60,6 +60,9 @@
     "dist",
     "parameter",
     "discrete",
+    "mean",
+    "average",
+    "avg",
     "negative",
     "binomial",
     "univariate"

lib/node_modules/@stdlib/stats/base/dists/negative-binomial/pmf/package.json

@@ -56,7 +56,7 @@
     "distribution",
     "dist",
     "probability",
-    "pdf",
+    "pmf",
     "density",
     "negative",
     "binomial",