fix: proofread Algebraic Topology I: Homotopy part

[vEnhance]

Proofreading pass over the Algebraic Topology I: Homotopy part (issue #315), covering the three chapters:

• tex/topology/constructions.tex (Some topological constructions)
• tex/topology/fundamental-group.tex (Fundamental groups)
• tex/topology/cover-project.tex (Covering projections)

All findings were minor (typos, grammar, one incorrect parenthetical). No large mathematical errors were found; the homotopy-groups-of-spheres table $\pi_n(S^m)$ was cross-checked and is correct.

Changes

• constructions.tex: added a missing period after "welding its boundary onto $\mathbb{CP}^{n-1}$".
• fundamental-group.tex: corrected "$S^1$ (the boundary of the unit circle)" → "(the unit circle)" — $S^1$ is the unit circle (it's the boundary of the unit disk, not of a circle).
• fundamental-group.tex: "generated by this loop $a$ and $b$ (and its inverses)" → "these loops $a$ and $b$ (and their inverses)".
• fundamental-group.tex: "instead of merely assigns some group" → "instead of merely assigning some group".
• cover-project.tex: fixed typo in codomain $\pi_1(Y_0, y_0)$ → $\pi_1(Y, y_0)$.
• cover-project.tex: "the lifting criteria is trivially satisfied" → "criterion" (singular).
• cover-project.tex: dropped a redundant trailing $\mathbb{Z}$ in "additive group structure $\mathbb{Z}$".

Part of the proofreading campaign (#315).

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Generated by Claude Code