Fix grammar and spelling issues in amss.md lecture

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This PR addresses grammar and spelling issues in the amss.md lecture file to improve readability while preserving all technical content and mathematical notation.

Key improvements made:

Grammar fixes:

• Added missing articles ("the") where appropriate
• Fixed subject-verb agreement: "AMSS allows" instead of "AMSS allow"
• Added proper comma usage: "Each period, a representative household"
• Changed informal tone to formal: "let's" → "we" in several instances
• Fixed tense consistency: "is also an implementability constraint"

Spelling and word choice:

• Corrected "facilities" to "facilitates"
• Improved "entries give probabilities" vs "tell probabilities"
• Fixed hyphenation: "flat-rate tax"

Formatting and spacing:

• Removed extra spaces in citations and throughout text
• Standardized mathematical notation spacing
• Cleaned up double spaces around punctuation

Examples of changes:

- Let's start with following imports:
+ Let's start with the following imports:

- A representative  household's preferences
+ The representative household's preferences

- AMSS allow the government to issue
+ AMSS allows the government to issue

- This pattern facilities smoothing
+ This pattern facilitates smoothing

All mathematical formulas, code blocks, references, and technical content remain exactly as before. The changes focus purely on improving the linguistic quality of the lecture while maintaining its academic rigor.

Fixes #232.

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