Fix PCA tutorial: remove extra spaces and add histogram instructions

[pahalsrivastava]

This PR updates the "Introduction to Remote Sensing" tutorial and addresses issue #104.

Changes made:

1. Fixed PCA command spacing

   • Removed extra spaces in CLI and Python i.pca commands so they run correctly.
   • Example CLI:

     i.pca input=landsat8_2024_B2,landsat8_2024_B3,landsat8_2024_B4,landsat8_2024_B5,landsat8_2024_B6,landsat8_2024_B7 output=Flagstaff_landsat8

   • Example Python:

     gs.run_command("i.pca", 
                    input="landsat8_2024_B2,landsat8_2024_B3,landsat8_2024_B4,landsat8_2024_B5,landsat8_2024_B6,landsat8_2024_B7",
                    output=Flagstaff_landsat8)

2. Added missing histogram instructions in Visualizing PCA results

   • Wrapped the bcyr color table step in a code block.
   • Added a missing step to reset histogram to grey for subsequent steps so the tutorial figures match.
   • Example code block added:

     # Set PCA component 1 to bcyr false color
     r.colors -e map=Flagstaff_landsat8.1 color=bcyr
     
     # Reset histogram to grey for subsequent steps
     r.colors -e map=Flagstaff_landsat8.1 color=grey

3. Wrapped all commands in code blocks for easier copy-pasting and clarity.

These changes make the tutorial easier to follow and ensure users can reproduce the figures correctly.