Fix a typo. (#27043)

`=== Instalation with Homebrew` should be `=== Installation with Homebrew`.

docs/reference/getting-started.asciidoc

@@ -149,7 +149,7 @@ And now we are ready to start our node and single cluster:
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 [float]
-=== Instalation with Homebrew
+=== Installation with Homebrew
 
 On macOS, Elasticsearch can also be installed via https://brew.sh[Homebrew]: