Fix some references to master not renamed to main

docs/advanced.rst

@@ -10,7 +10,7 @@ Control of unpaper
 
 OCRmyPDF uses ``unpaper`` to provide the implementation of the
 ``--clean`` and ``--clean-final`` arguments.
-`unpaper <https://github.com/Flameeyes/unpaper/blob/master/doc/basic-concepts.md>`__
+`unpaper <https://github.com/Flameeyes/unpaper/blob/main/doc/basic-concepts.md>`__
 provides a variety of image processing filters to improve images.
 
 By default, OCRmyPDF uses only ``unpaper`` arguments that were found to

docs/installation.rst

@@ -637,7 +637,7 @@ system-wide:
 
 .. code-block:: bash
 
-    git clone -b master https://github.com/ocrmypdf/OCRmyPDF.git
+    git clone -b main https://github.com/ocrmypdf/OCRmyPDF.git
     python3 -m venv venv
     source venv/bin/activate
     cd OCRmyPDF
@@ -664,7 +664,7 @@ To install all of the development and test requirements:
 
 .. code-block:: bash
 
-    git clone -b master https://github.com/ocrmypdf/OCRmyPDF.git
+    git clone -b main https://github.com/ocrmypdf/OCRmyPDF.git
     python -m venv
     source venv/bin/activate
     cd OCRmyPDF

docs/languages.rst

@@ -12,7 +12,7 @@ OCRmyPDF uses Tesseract for OCR, and relies on its language packs for all langua
 On most platforms, English is installed with Tesseract by default, but not always.
 
 Tesseract supports `most
-languages <https://github.com/tesseract-ocr/tesseract/blob/master/doc/tesseract.1.asc#languages>`__.
+languages <https://github.com/tesseract-ocr/tesseract/blob/main/doc/tesseract.1.asc#languages>`__.
 Languages are identified by standardized three-letter codes (called ISO 639-2 Alpha-3).
 Tesseract's documentation also lists the three-letter code for your language.
 Some are anglicized, e.g. Spanish is ``spa`` rather than ``esp``, while others