docs(install): fix link to the ".NET tool" section

At least in GitHub-flavored Markdown, that section title gets
transformed into the anchor `net-tool`, not `.NET-tool`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

docs/install.md

@@ -64,7 +64,7 @@ sudo /usr/local/share/gcm-core/uninstall.sh
 
 ### .NET tool :star:
 
-See the [.NET tool](#.NET-tool) section below for instructions on this
+See the [.NET tool](#net-tool) section below for instructions on this
 installation method.
 
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