hardening docs: fix typo

Changed files
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doc
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doc/stdenv.xml
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</para>
<para>Addtionally, some warnings are enabled which might trigger build
-
failures if compiler warnings are treated as errors in the packsge build.
+
failures if compiler warnings are treated as errors in the package build.
In this case, set <option>NIX_CFLAGS_COMPILE</option> to
<option>-Wno-error=warning-type</option>.</para>