[kalman_2] update exposition of first three sections

[longye-tian]

Hi @jstac

This PR makes a few small notation and exposition updates to kalman_2.md following #902

I would be grateful if you could have a look when you have time.

• Fixes a couple of opening typos.
• Clarifies notation in the worker-output model:
  • renames the human-capital shock from w_{t+1} to \epsilon_{t+1};
  • defines \epsilon_{t+1};
  • standardizes normal distribution notation to N following the style guide
  • aligns the parameter list with \sigma_{h,0} and \sigma_{u,0};
  • explains that u_{t+1} = u_t treats effort as a fixed, unobserved component of worker type.
• Clarifies that the firm sets the wage using past output history y^{t-1} before observing current output y_t.
• Standardizes expectation notation to \mathbb{E} in the relevant text and figure labels following the style guide

Best,
Longye

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[jstac]

Many thanks @longye-tian !

Just one small suggestion.

[longye-tian]

Many thanks @jstac !

Just updated all three instances of them in this lecture.

[jstac]

Many thanks @longye-tian , merging.