docs: add domain constraint to @stdlib/stats/base/dists/chisquare/mean intro

The intro section previously read "where \`k\` is the degrees of
freedom"; updated to "where \`k >= 0\` is the degrees of freedom" to
match the convention used by the three other `NonNegativeNumber`-domain
siblings in this namespace (`mode`, `stdev`, `variance` — 3/4 = 75%
conformance). The constraint matches the existing Notes statement
"If provided \`k < 0\`, the function returns \`NaN\`" found later in
the same file.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/chisquare/mean/README.md

@@ -41,7 +41,7 @@ The [expected value][expected-value] for a [chi-squared][chisquare-distribution]
 
 <!-- </equation> -->
 
-where `k` is the degrees of freedom.
+where `k >= 0` is the degrees of freedom.
 
 </section>