Reference main not master in install link (#4445)

Author: connorjward

docs/source/install.rst

@@ -54,7 +54,7 @@ firedrake-configure
 To simplify the installation process, Firedrake provides a utility script called
 ``firedrake-configure``. This script can be downloaded by executing::
 
-  $ curl -O https://raw.githubusercontent.com/firedrakeproject/firedrake/master/scripts/firedrake-configure
+  $ curl -O https://raw.githubusercontent.com/firedrakeproject/firedrake/main/scripts/firedrake-configure
 
 Note that ``firedrake-configure`` **does not install Firedrake for you**. It
 is simply a helper script that emits the configuration options that Firedrake