Improve COBOL snippets in doc, and improve code readability in COBOL sources

[vargajb]

Proposed changes

Fix syntax errors in COBOL snippets in doc

• "WORKING-STORAGE." -> "WORKING-STORAGE SECTION."
• "WORKING-STORAGE SECTION" -> "WORKING-STORAGE SECTION."
• "PROCEDURE DIVISION" -> "PROCEDURE DIVISION."

Improve loop readability in COBOL programs and in COBOL snippets in doc

• Old

PERFORM UNTIL LASTREC = 'Y'
PERFORM WRITE-RECORD
PERFORM READ-RECORD
END-PERFORM
.....
READ ACCT-REC
AT END MOVE 'Y' TO LASTREC
END-READ.

• Improved

PERFORM UNTIL LASTREC = 'Y'
    PERFORM WRITE-RECORD
    PERFORM READ-RECORD
END-PERFORM
.....
READ ACCT-REC
    AT END MOVE 'Y' TO LASTREC
END-READ.

Fixes # (issue)

Type of change

What type of changes does your PR introduce to the COBOL Programming Course? Put an x in the boxes that apply.

• Bug fix (change which fixes one or more issues)
• New feature (change which adds functionality or features to the course)
• Translations (change which adds or modifies translations of the course)
• Documentation (change which modifies documentation related to the course)
• This change requires an update to the course's z/OS environment

Checklist:

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• I have read the Contributing Guideline
• I have included a title and description for this PR
• I have DCO-signed all of my commits that are included in this PR
• I have tested it manually and there are no regressions found
• I have commented my code, particularly in hard-to-understand areas (if appropriate)
• I have made corresponding changes to the documentation (if appropriate)

[tanto259]

Thanks!