Fix pip-based installation instructions on Windows (double quotes) (#462)

On Windows, single quotes are not interpreted as argument grouping, but
rather literal characters, which breaks installation instructions.

README.md

@@ -34,10 +34,10 @@ pipx creates an isolated environment for each program, and adds their binaries i
 
 - Install Python 3.8 or above.
 - Install pipx using either your package manager, `pip3 install --user pipx`, or `pip install --user pipx`.
-- Run `pipx install 'corrscope[qt5]'`
-    - On Linux, to add support for native Qt themes, instead run `pipx install --system-site-packages 'corrscope[qt5]'`
-    - On FreeBSD, install `py-qt5` and `py39-ruamel.yaml` via `pkg` or ports tree, and install `ffmpeg` via ports tree with `SDL` option enabled. After everything's installed, run `pipx install --system-site-packages 'corrscope[qt5]'`
-    - On M1 Mac, instead run `pipx install 'corrscope[qt6]'`
+- Run `pipx install "corrscope[qt5]"`
+    - On Linux, to add support for native Qt themes, instead run `pipx install --system-site-packages "corrscope[qt5]"`
+    - On FreeBSD, install `py-qt5` and `py39-ruamel.yaml` via `pkg` or ports tree, and install `ffmpeg` via ports tree with `SDL` option enabled. After everything's installed, run `pipx install --system-site-packages "corrscope[qt5]"`
+    - On M1 Mac, instead run `pipx install "corrscope[qt6]"`
 - Open a terminal and run `corr (args)`.
 
 ### Installing from PyPI via Pip (cross-platform, releases)
@@ -46,8 +46,8 @@ pip installs packages into a per-user Python environment. This has the disadvant
 
 - Install Python 3.8 or above.
 - If necessary, install pip using your package manager.
-- Run `pip3 install --user 'corrscope[qt5]'`
-    - On M1 Mac, instead run `pip3 install --user 'corrscope[qt6]'`
+- Run `pip3 install --user "corrscope[qt5]"`
+    - On M1 Mac, instead run `pip3 install --user "corrscope[qt6]"`
 - Open a terminal and run `corr (args)`.
 
 ### Dev builds (Windows)