Update coverage.rst

The header for the -d option says '-hist', I edited the heading to say '-d' instead.

docs/content/tools/coverage.rst

@@ -143,7 +143,7 @@ been reported, a histogram summarizing the coverage among all features in B will
 
 
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-``-hist`` Reporting the per-base of coverage for each feature in the B file 
+``-d`` Reporting the per-base of coverage for each feature in the B file 
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 One should use the "**-d**" option to create, for each interval in B, a detailed list of coverage at each of the
 positions across each B interval.