Update CLI helpdoc formatting to allow indentation in code

Use `textwrap.dedent()` to allow indented cli-helpdoc in `__main__.py` file. The indentation increases readability, while `textwrap.dedent` helps maintain the same functionality without breaking code.

src/markitdown/__main__.py

@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: MIT
 import sys
 import argparse
+from textwrap import dedent
 from ._markitdown import MarkItDown
 
 
@@ -10,7 +11,7 @@ def main():
     parser = argparse.ArgumentParser(
         description="Convert various file formats to markdown.",
         formatter_class=argparse.RawDescriptionHelpFormatter,
-        usage="""
+        usage=dedent("""
             SYNTAX: 
                 
                 markitdown <OPTIONAL: FILENAME>
@@ -27,7 +28,7 @@ EXAMPLE:
                 OR 
             
                 markitdown < example.pdf
-""".strip(),
+            """).strip(),
     )
 
     parser.add_argument("filename", nargs="?")